Vehicle evaporative emission control systems may be configured to store fuel vapors from fuel tank refueling and diurnal engine operations, and then purge the stored vapors during a subsequent engine operation. In an effort to meet stringent federal emissions regulations, emission control systems may need to be intermittently diagnosed for the presence of undesired evaporative emissions that could release fuel vapors to the atmosphere.
In one example, an evaporative emissions test diagnostic procedure utilizes engine vacuum to evacuate the evaporative emissions control system and a vehicle fuel system to a target vacuum (e.g., −8 InH2O) during vehicle cruising conditions, where vehicle cruising conditions may comprise a steady state vehicle speed greater than forty miles-per-hour, for example. Responsive to the target vacuum being reached, the evaporative emissions control system and fuel system may be sealed from atmosphere, and a pressure bleed-up may be monitored. A pressure bleed-up rate greater than a predetermined pressure bleed-up rate, or if pressure in the fuel system and evaporative emissions control system reaches a level greater than a predetermined pressure threshold, undesired evaporative emissions may be indicated. However, in some examples it may be difficult to distinguish between undesired evaporative emissions or whether the observed pressure bleed-up is a result of fuel vaporizing due to hot engine exhaust. As such, undesired evaporative emissions may be wrongly indicated under circumstances where large pressure bleed-up occurs due to fuel vaporization effects.
Other attempts to address the difficulties in interpreting whether pressure bleed-up is due to fuel vaporization effects or due to actual undesired evaporative emissions include running the evaporative emissions test diagnostic during cold start conditions. One example approach is shown by Dawson et al. in U.S. Pat. No. 6,530,265. Therein, a method is taught whereby it is first determined whether cold start conditions are met prior to initiating an evaporative emissions test diagnostic utilizing engine vacuum to evacuate the evaporative emissions control system and fuel system. By initiating the test diagnostic under cold start conditions, it is taught that the fuel system may be stable for testing. However, the inventors herein have recognized potential issues with such methods. As one example, such a method may result in undesired emissions due to an exhaust catalyst being below a threshold temperature (e.g., light-off temperature) for oxidation of unburnt hydrocarbons. Specifically, evaporative emissions control systems typically include a fuel vapor canister with a buffer region between a load port of the canister, and the purge port of the canister. The buffer functions to prevent fuel tank vapors from entering the engine directly, and as such, the buffer acts as a vapor filter. At a key-off event, the buffer is typically clean from vapors due to purging events during a previous drive cycle. However, during a soak condition, the buffer may again be loaded from diurnal fuel tank vapors in addition to vapor migration within the canister itself. As such, if a cold-start evaporative emissions test diagnostic is initiated when the buffer is full, undesired emissions may result due to the catalyst being below the threshold temperature.
Thus, the inventors herein have developed systems and methods to at least partially address the above issues. In one example, a method is provided, comprising during a first operating mode, routing fuel vapors from a fuel tank through a first vapor storage device into an intake manifold of an internal combustion engine; and during a second operating mode, routing fuel vapors from the fuel tank through a second vapor storage device but not through the first vapor storage device. Such modes may be accomplished via a first vapor storage device being separated from a second vapor storage device by a one way vacuum-actuated check valve.
As one example, during the first operating mode, fuel vapors from the fuel tank and not from the second fuel vapor storage device are routed through the first vapor storage device. As such, during an engine cold start event where an exhaust catalyst is below a temperature required for catalytic activity, an evaporative emissions test diagnostic may be conducted using engine intake manifold vacuum to evacuate a fuel system and evaporative emissions control system, wherein fuel vapors from the fuel tank are adsorbed by the first fuel vapor storage device. In this way, the evaporative emissions test may be conducted under cold start conditions, without an increase in undesired exhaust emissions during the cold start event, and wherein the results of the evaporative emissions test are not complicated by the effects of fuel vaporization on pressure in the fuel system and evaporative emissions control system.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.